1. Field of the Invention
The present invention relates to a power supply in an electronic apparatus such as an image forming apparatus, and a circuit board and a fixing structure for fixing a heatsink (radiator) for radiating heat generated by an electronic component, such as a power transistor installed on the circuit board, which are applicable to the power supply.
2. Description of the Related Art
In a power supply for supplying electric power to each unit of an electronic apparatus, such as an image forming device, it is necessary to supply relatively large amounts of electric power stably, so that an electronic component forming a circuit such as a power transistor generates a large amount of heat. Thus, a heatsink serving as a radiator for radiating the heat generated in the electronic component is provided on a circuit board of the power supply, so as to protect the electronic component against the heat.
FIGS. 5 to 7 show a heatsink used in a circuit board of a conventional power supply and a fixing structure of the heatsink. The power supply has a circuit board 101 on which a wiring pattern is formed, an electronic component 102, such as a power transistor, installed on the circuit board 101 in an upright position, a heatsink 103 attached to the circuit board 101 in an upright position and a conductor 120 for electrically connecting a grounding line of the circuit board 101 to the heatsink 103. The heatsink 103 is formed by bending an aluminum plate in a substantially right-angular cornered U-letter shape, and the electronic component 102 is closely mounted on a component attachment face 105 of both opposed faces of the heatsink 103. The conductor 120 made of iron is attached to a back face 106 opposed to the component attachment face 105. A bottom face 107, which couples the component attachment face 105 to the back face 106, is brought into contact with the circuit board 101.
The electronic component 102 is attached to the heatsink 103 at substantially the center thereof by use of a screw 108. In order to prevent interference with terminals of the electronic component 102, the component attachment face 105 of the heatsink 103 is cut on the periphery of the terminals. A convex portion 121, which comes into contact with a tip of the conductor 120, is provided on the back face 106 of the heatsink 103, and a bearing portion 122 for supporting a shaft of the conductor 120, is provided between the convex portion 121 and the bottom face 107. The convex portion 121 and the bearing portion 122 are formed by press working, for example. A fixing portion 123, which protrudes in a shape of a hook for fixing the heatsink 103 on the circuit board 101, is provided at bottom ends of a pair of opposed corner portions on the component attachment face 105 and the back face 106. The fixing portion 123 is inserted into a rectangular hole 124 provided on the circuit board 101, and then is twisted while being held by a tool such as pliers prior to a dipping process, thereby engaging with the periphery of the rectangular hole 124 to fix the heatsink 103 to the circuit board 101. FIG. 5 shows the state where the fixing portion 123 is inserted into the rectangular hole 124, and FIG. 7 shows the state where the twisted fixing portion 123 engages with the periphery of the rectangular hole 124.
A front end of the conductor 120 comes into contact with the convex portion 121, and the other front end thereof penetrates through a through hole 125 formed on the circuit board 101 and is soldered to a land portion 111 provided on the circuit board 101. The land portion 111 is connected to the grounding line of the circuit board 101, so that the heatsink 103 is electrically connected to the grounding line of the circuit board 101 via the conductor 120. Soldering of the conductor 120 is performed by being dipped into a molten solder tank simultaneously with soldering of the electronic component 102.
In such a conventional power supply, it is adopted that the heatsink 103 is fixed on the circuit board 101 by engaging the twisted fixing portion 123 with the periphery of the rectangular hole 124. The heatsink 103 made of aluminum, however, cannot be directly fixed on the circuit board 101 by solder, so that there may be cases where the heatsink 103 cannot be completely fixed on the circuit board 101. Furthermore, since the heatsink 103 is formed of an aluminum plate, it is easily deformed. When the heatsink 103 is subjected to shock or vibration for any reason, a clearance may occur between the fixing portion 123 and the periphery of the rectangular hole 124. In such a case, the contact between the conductor 120 and the heatsink 103 becomes unstable, so that it causes a conduction failure and the operation of the power supply becomes unstable. Furthermore, the process of bringing the bottom face 107 of the heatsink 103 into close contact with the circuit board 101 while stably contacting the conductor 120 against the convex portion 121, and twisting the fixing portion 123 being held by pliers or the like, requires some experience and imposes a burden on the operator.
On the other hand, Japanese Laid-Open Utility Model Publication No. 6-41192 discloses a grounding metal fitting having the same function as the conductor 120 in the power supply shown in FIGS. 5 to 7. However, the ground metal portion cannot solve the above-mentioned problems. In addition, Japanese Laid-Open Patent Publication No. 2003-46027 discloses a heat radiation configuration of a ball grid array.